1. Field of the Invention
The present invention relates to a polymerization solution having a small environmental load, having excellent economic efficiency and providing a conductive polymer exhibiting excellent heat resistance. The present invention also relates to a conductive polymer film, a polymer electrode and a solid electrolytic capacitor obtained from the polymerization solution.
2. Description of the Related Art
Conductive polymers delivered from thiophenes have been known in the prier art, and have been applied to a wide range of fields such as solid electrolytic capacitors, polymer batteries, antistatic films, indicating elements, sensors and electrode materials. Especially, the application of conductive polymers obtained from 3,4-disubstituted thiophenes including 3,4-ethylenedioxythiophene (hereinafter 3,4-ethylenedioxythiophene is referred to as “EDOT” and poly(3,4-ethylenedioxythiophene) is referred to as “PEDOT”) has been frequently researched because these polymers have high conductivity and environmental durability.
These conductive polymers can be obtained by an electrolytic polymerization method or a chemical polymerization method (for example, see Patent Document 1 (JP 1-313521 A). According to the electrolytic polymerization method, a conductive polymer film with excellent mechanical strength can be formed in a short period of time from a small quantity of monomers on an electrode. The conductive polymer film formed on the electrode is used in a form where it is peeled from the electrode, or it is also used in a form where it is placed on the electrode as a polymer electrode. In addition, Patent Document 2 (JP 2-15611 A), for example, suggests using a polymer delivered from a 3,4-disubstituted thiophene including EDOT as an conductive polymer layer in a solid electrolytic capacitor that contains a positive electrode made of valve metal foil such as aluminum foil, tantalum foil and niobium foil that has an oxide film as a dielectric on the surface, and an conductive polymer layer that is adjacent to the oxide film and acts as a real negative electrode.
Electrolytic polymerization of thiophenes is normally carried out with a polymerization solution that contains an organic solvent such as ethanol and acetonitrile. This is because the solubility of thiophenes in water is remarkably low and because polymerization is considered to be inhibited by water because the oxidation potential of thiophenes is higher than that of water. For example, in working examples 9 to 14, and 18 to 22 of Patent Document 1, PEDOT is obtained by electrolytic polymerization using a polymerization solution that contains acetonitrile as a solvent and EDOT as a monomer. Also, in a working example 4 of Patent Document 3 (JP 2-58818 A), a solid electrolytic capacitor is obtained by electrolytic polymerization using a polymerization solution that contains acetonitrile as a solvent, diisopropylammonium borodisalicylate as a supporting electrolyte, and 3,4-dimethylthiophene as a monomer. This document also states that a polymerized film with a problem in terms of thermal stability is obtained from a polymerization solution that is prepared by using ammonium borodisalicylate, which has low solubility in an organic solvent, as a supporting electrolyte, and adding water to an organic solvent.
However, the use of an organic solvent generally increases environmental burdens and is economically disadvantageous compared with the use of water. Moreover, many organic solvents are harmful to humans, and in the case of electrolytic polymerization using a combustible solvent, procedures need to be taken to prevent fire by electric sparks. Therefore, for polymerization of thiophenes that dissolve sparingly in water, consideration has been given to using water, which has small environmental burdens and is highly economical, as a solvent, and also to a method to use an anionic surfactant or polymerized anionic surfactant with a sulfonic acid group or a sulfonic acid salt group, and to a method to using an inorganic supporting electrolyte as a supporting electrolyte in the polymerization solution for electrolytic polymerization.
For example, Patent Document 4 (JP 2000-269087 A) discloses a method of preparing a polymer of a thiophene derivative on a positive electrode for a solid electrolytic capacitor, by forming a conductive layer on an oxide film of the positive electrode by chemical polymerization or other methods, introducing this positive electrode in an aqueous polymerization solution in which the thiophene derivative such as EDOT is emulsified by a alkylnaphthalenesulphonate surfactant, and performing electrolytic polymerization. Since the thiophene derivative is condensed in a micelle of the surfactant, polymerization proceeds rapidly, and a conductive polymer layer, in which an anion of the surfactant is doped, is formed on the positive electrode. Because of the bulkiness of an alkylnaphthalenesulfonate anion, which is taken into the polymer layer as a dopant by electrolytic polymerization, de-doping is inhibited, and a solid electrolytic capacitor, which is stable under high temperature and high humidity, is obtained.
Also, Non-patent Document 1 (Synthetic Met. (2009), 159 (5-6) 406-414) reports electrolytic polymerization using an aqueous solution in which EDOT and sodium polystyrene sulfonate as a polymerized anionic surfactant are dissolved. Solubility of EDOT in water is increased by the action of sodium polystyrene sulfonate, EDOT is more prone to be oxidized, and a uniform PEDOT film is formed on a Pt electrode. Patent Document 5 (JP 2006-295184 A) does not relate to electrolytic polymerization, but discloses a method of preparing a conductive polymer on a positive electrode for a solid electrolytic capacitor by forming a conductive layer on an oxide film of the positive electrode by a chemical polymerization method or other methods and then applying on the conductive layer a dispersion that contains conductive polymer particles and a binding agent. The specifically disclosed dispersion is obtained by adding EDOT, iron (III) sulfate and sodium persulfate in an aqueous solution containing polystyrene sulfonic acid that acts as a polymerized anionic surfactant, forming PEDOT/polystyrene sulfonate particles by their reaction in the aqueous solution, and then adding a binding agent and other agents.
Moreover, Non-patent Document 2 (Electrochimica Acta 48 (2003) 1713-1717) reports electrolytic polymerization by the potentiostatic polymerization method in an aqueous solution that contains 0.01 M EDOT and 0.1 M lithium perchlorate as a supporting electrolyte, and Non-patent Document 3 (Synthetic Met. (2010), 160 (15-16), 1636-1641) reports electrolytic polymerization by the potential sweep method in an aqueous solution that contains 0.01 M EDOT and 0.1 M potassium nitrate as a supporting electrolyte.